1. Field of Invention
This invention relates to a subscriber line tester for communication lines, such as telephone lines, that convey high speed digital data; and more particularly, to such a tester that allows high speed communication and measurement over a long period of time.
2. Description of the Prior Art
With conventional communication lines, it is desired to measure various transmission characteristics thereof, such as attenuation, cross talk, and noise. These characteristics are measured by supplying a signal from one end of a "loop under test" of the communication line (called "loop under test") to be tested, and by measuring the signal at the other end of the loop under test.
FIG. 1 shows a conventional subscriber line tester comprising a signal generator 1, such as a tracking oscillator or a sweeping oscillator, connected to one end of a loop under test 100, and a measuring equipment 2, such as a selective level meter or a spectrum analyzer, connected to the other end of the loop under test 100.
Operation of the conventional tester will be described with reference to FIGS. 2 and 3, wherein FIG. 2 shows the attentuation characteristics of the loop under test 100, and FIG. 3 shows the noise characteristics of the loop under test 100. For example, if the attentuation characteristics of the loop under test 100 is to be measured, the frequency of the signal to be outputted to the loop under test 100 is subjected to sweeping by signal generator 1. That signal is propagated through the loop under test 100, and at the other end thereof, the propagated signal is captured by the measuring equipment 2 to obtain a spectrum which is displayed on a display device or the like. The attenuation characteristic shown in FIG. 2 is obtained by such measurement.
In a similar manner, the noise characteristic of the loop under test 100 is measured by supplying a signal to the loop under test 100 using signal generator 1, and then measuring the noise signal input propagated through the loop under test 100 using the measuring equipment 2, and obtaining a spectrum which is displayed in the display means. The noise characteristic shown in FIG. 3 is thus obtained by such measurement.
However, for the subscriber line tester shown in FIG. 1, there is a problem in that the measuring time period is prolonged if the measuring frequency range is wide when measurement is done with the measurement equipment 2 and the signal generator 1. This is because of the necessity to measure levels by automatically capturing selected frequencies or to measure the peaks of a spectrum of varying signal frequency in the maximum value retaining mode when a selective level meter or a spectrum analyzer is used in the measuring equipment 2.
Another problem is that measurement of the transmission characteristic is not simple when the loop under test 100 is long in length, such as being several kilometers, since the loop under test is the loop actually being tested. This is due to the fact that one group of workers must be present at the signal generator 1 location, and another group of workers must be present at the measuring equipment 2 location, and that these two groups must be in communication with each other so that the two groups can simultaneously operate the respective apparatus on both ends of the loop under test 100 when measuring transmission characteristics thereof.
Moreover, an other problem is that when noise is estimated, the accuracy of such estimation depends on human skill at judging the noise spectra. The noise being estimated are often the result of cross talk between communication lines. Thus, noise due to cross talk, for example, requires a better form of measurement.
Furthermore, when noise is generated in a non-steady manner, it is necessary to measure the noise of a long period of time. This creates another problem in that a large amount of data is accummulated as the measurement time elapses. Such accummulation is wastesful and requires a larger memory capacity than otherwise necessary. The amount of accumulated data can be reduced if the measurement is made in a fixed cycle. But, disadvantageously, a non-steady, or random noise might be missed in that case, that is such noise might not be measured properly.